1. Field of the Invention
The present invention relates to an overcurrent detection circuit, in particular to a technique for improving the accuracy of current detection.
2. Description of Related Art
In recent years, replacements from mechanical relays in the related art to power MOSFETs equipped with control circuits, i.e., IPDs (Intelligent Power Devices) as switch elements for driving loads such as lamps and motors in vehicles such as cars have been in progress in order to achieve higher reliability/lower on-resistance/lower costs. In a system provided with a load and an IPD, for example, if an abnormal condition such as short-circuit of wiring or the load occurs and thereby an overcurrent flows into the load, there is a possibility that a power MOSFET provided in the IPD and the load will be damaged. It has been therefore common practice to provide the IPD with a circuit that detects an overcurrent and turns off the power MOSFET before these components are damaged, i.e., with an overcurrent detection circuit. Further, it has been also desired that current-detection-value characteristics of such overcurrent detection circuits have high accuracy so that the loads and the power MOSFETs are protected more safely. That is, it has been desired to reduce errors caused by variations in the characteristics of each component.
Japanese Unexamined Patent Application Publication No. 2005-039573 proposes a solution for this problem. FIG. 9 shows a load drive circuit (overcurrent detection circuit) disclosed in Japanese Unexamined Patent Application Publication No. 2005-039573. The circuit shown in FIG. 9 includes a power-supply voltage terminal 1, a load 2, an input terminal 3, an output terminal 4, a control circuit 5, a ground voltage terminal 6, constant-current output means 9, threshold current output means 10, an output MOS transistor Q1, a current detection MOS transistor Q2, a detection voltage transfer MOS transistor Q3, a detection signal output MOS transistor Q4, and a detection resistor Rs. Note that the transistors Q2, Q3 and Q4, the control circuit 5, the detection resistor Rs, the constant-current output means 9, and the threshold current output means 10 constitute a load drive circuit. Note that the circuit in FIG. 9 has a function for detecting an overcurrent between the source and drain of the transistor Q1 when the power-supply voltage is supplied from the power-supply voltage terminal 1 to the load 2. In particular, the circuit in FIG. 9 is characterized in that it can detect an overcurrent even when the potential at the output terminal 4 is lower than the potential at the ground voltage terminal.
The structure of the circuit shown in FIG. 9 is briefly explained hereinafter. The power-supply voltage terminal 1 is connected through the input terminal 3 to the drains of Q1 and Q2, and to the input terminals of the constant-current output means 9 and the threshold current output means 10. The power-supply voltage terminal on the high-potential side of the load 2 is connected through the output terminal 4 to a node 8. The node 8 is also connected to the source of the transistor Q1, one terminal of the detection resistor Rs, and the source of the transistor Q4. Furthermore, the power-supply voltage terminal on the low-potential side of the load 2 is connected to the ground voltage terminal 6.
The output terminal of the control circuit 5 is connected to the gates of the transistors Q1 and Q2. The source of the transistor Q2 is connected to a node 7. The node 7 is also connected to the other terminal of the detection resistor Rs and the source of the transistor Q3. The output terminal of the constant-current output means 9 is connected to a node 11. The note 11 is also connected to the drain and the gate of the transistor Q3 and the gate of the transistor Q4. The output terminal of the threshold current output means 10 is connected to a node 12. The node 12 is also connected to the drain of the transistor Q4 and the output terminal from which an overcurrent detection signal is output.
Next, the operations of the circuit in FIG. 9 are explained hereinafter. The switching between On/Off states of the power-supply voltage supplied from the power-supply voltage terminal 1 to the load 2 is controlled by the transistor Q1. That is, the connection between the source and drain of the transistor Q1 is controlled by a control signal output from the control circuit 5.
Since the transistors Q1 and Q2 are structurally similar to each other (only the dimensions are different, and characteristics per unit channel width are equivalent), a current flowing through the transistor Q2 increases with the increase in a current flowing through the transistor Q1 based on the homothetic ratio between the transistors Q1 and Q2 (for example, if the current flowing through the transistor Q1 is 10 A and the homothetic ratio is 10000:1, the current flowing through the transistor Q2 becomes 10 A/10000=1 mA). As a result, the potential Vs at the node 7 and the potential V1 at the node 11 rise. That is, when the transistor Q4 is turned on, the current flowing therethrough becomes larger. Note that the transistors Q3 and Q4 are structurally similar to each other.
When a current flowing between the source and drain of this transistor Q4 exceeds a threshold current Iref2 (e.g., 50 uA) established by the threshold current output means 10, an overcurrent detection signal output through the node 12 is inverted from the high level to the low level. Therefore, the load drive circuit can determine that the current flowing into the load is in an overcurrent state.
On the other hand, when the current flowing through the transistor Q1 is small, the current that flows when the transistor Q4 is turned on is smaller than the threshold current Iref2. At this time, the overcurrent detection signal output through the node 12 remains in the high-level state. Therefore, the load drive circuit can determine that the current flowing into the load is not in an overcurrent state.
Note that as shown in Japanese Unexamined Patent Application Publication No. 2005-039573, in addition to the relation between the transistors Q1 and Q2 and the relation between the transistors Q3 and Q4, the constant-current output means 9, which outputs the signal Iref1, and the threshold current output means 10, which outputs the signal Iref2, are also structurally similar to each other.
We had examined the operations of the circuit shown in FIG. 9 in detail. To explain the circuit in FIG. 9, an assumption is made that the current flowing through Q1 is Ioc, the current flowing through Q2 is Isense, the homothetic ratio between Q1 and Q2 is A:1, the channel length of Q3 is L1, the channel width of Q3 is w1, the channel length of Q4 is L2, the channel width of Q4 is w2, the threshold voltage of Q3 and Q4 is Vt, the mobility of electrons is μ, and the oxide film capacitance per unit area is Cox. Note that Isense is sufficiently larger than Iref1. In the circuit of FIG. 9, the current value Ioc detected by the load drive circuit can be expressed by the following Equation (1).
                                          V            ⁢                                                  ⁢            1                    =                    ⁢                                    Isense              ·                                                          ⁢              Rs                        +                                                                                2                    ⁢                    L                    ⁢                                                                                  ⁢                    1                                                                              μ                      ·                      Cox                      ·                      w                                        ⁢                                                                                  ⁢                    1                                                  ⁢                Iref                ⁢                                                                  ⁢                1                                      +            Vt                                                        =                    ⁢                                                                                          2                    ⁢                    L                    ⁢                                                                                  ⁢                    2                                                                              μ                      ·                      Cox                      ·                      w                                        ⁢                                                                                  ⁢                    2                                                  ⁢                Iref                ⁢                                                                  ⁢                2                                      +            Vt                                    Isense    =                  1        Rs            ⁢                        2                      μ            ·            Cox                              ⁢              (                                                                              L                  ⁢                                                                          ⁢                  2                                                  w                  ⁢                                                                          ⁢                  2                                            ⁢              Iref              ⁢                                                          ⁢              2                                -                                                                      L                  ⁢                                                                          ⁢                  1                                                  w                  ⁢                                                                          ⁢                  1                                            ⁢              Iref              ⁢                                                          ⁢              1                                      )            Since Ioc=A·Isense, the following equation is obtained.
                    Ioc        =                              A            Rs                    ⁢                                    2                              μ                ·                Cox                                              ⁢                      (                                                                                                      L                      ⁢                                                                                          ⁢                      2                                                              w                      ⁢                                                                                          ⁢                      2                                                        ⁢                  Iref                  ⁢                                                                          ⁢                  2                                            -                                                                                          L                      ⁢                                                                                          ⁢                      1                                                              w                      ⁢                                                                                          ⁢                      1                                                        ⁢                  Iref                  ⁢                                                                          ⁢                  1                                                      )                                              (        1        )            
Assuming that, in Equation (1), the variation coefficient of Iref1 is x, the variation coefficient of Iref2 is y, and the variation coefficient of Rs is z, it can be expressed as the following Equation (2). Note that if the variation coefficient is 1, its variation characteristic indicates a standard value.
                    Ioc        =                              A                          Rs              ·              z                                ⁢                                    2                              μ                ·                Cox                                              ⁢                      (                                                                                                      L                      ⁢                                                                                          ⁢                      2                                                              w                      ⁢                                                                                          ⁢                      2                                                        ⁢                  Iref                  ⁢                                                                          ⁢                                      2                    ·                    y                                                              -                                                                                          L                      ⁢                                                                                          ⁢                      1                                                              w                      ⁢                                                                                          ⁢                      1                                                        ⁢                  Iref                  ⁢                                                                          ⁢                                      1                    ·                    x                                                                        )                                              (        2        )            Note that since the constant-current output means 9 and the threshold current output means 10 are structurally similar to each other in the related art, variation coefficients of Iref1 and Iref2 are equal. That is, an equation x=y is satisfied. Therefore, Equation (2) can be expressed as follows.
      Ioc    ⁡          (              x        ,        z            )        =            A              Rs        ·        z              ⁢                  2                  μ          ·          Cox                      ⁢          (                                                                  L                ⁢                                                                  ⁢                2                                            w                ⁢                                                                  ⁢                2                                      ⁢            Iref            ⁢                                                  ⁢                          2              ·              x                                      -                                                            L                ⁢                                                                  ⁢                1                                            w                ⁢                                                                  ⁢                1                                      ⁢            Iref            ⁢                                                  ⁢                          1              ·              x                                          )      Accordingly, the variation coefficient of Ioc is expressed as follows.
                                          Ioc            ⁡                          (                              x                ,                z                            )                                            Ioc            ⁡                          (                              1                ,                1                            )                                      =                              x                    z                                    (        3        )            
In this example, if each component has a variation within ±20%, i.e., if the variations occur within a range of x=0.8-1.2 and z=0.8-1.2, the maximum value and the minimum value of the variation coefficient of Ioc can be expressed as follows.
            maximum      ⁢                          ⁢      value      ⁢              :            ⁢                          ⁢                        Ioc          ⁡                      (                          x              ,              z                        )                                    Ioc          ⁡                      (                          1              ,              1                        )                                =                            1.2                0.8            =                        1.3693          ⁢                                          ⁢          …                ⁢                                  ≈        1.369                        minimum      ⁢                          ⁢      value      ⁢              :            ⁢                          ⁢                        Ioc          ⁡                      (                          x              ,              z                        )                                    Ioc          ⁡                      (                          1              ,              1                        )                                =                            0.8                1.2            =                        0.7454          ⁢                                          ⁢          …                ⁢                                  ≈        0.745            That is, the current value Ioc detected by the load drive circuit (the overcurrent detection value) exhibits a large variation with a range of +36.9% to −25.5% (variation width=62.4%).